1. Field of Invention
The present invention relates to a thin film transistor and manufacturing method thereon. More particularly, the present invention relates to a polysilicon thin film transistor and method of forming the same.
2. Description of Related Art
Due to rapid progress in electronic technologies, digitized video or imaging device has become an indispensable product in our daily life. Among the video or imaging products, displays are the principle devices for providing information. Through a display device, a user is able to obtain information or to control various operations. To facilitate the users, most video or imaging equipment is now designed with a slim and fairly light body. With breakthroughs in opto-electronic technologies and advances in semiconductor fabrication techniques, flat panel type of displays such as a thin film transistor liquid crystal display (TFT-LCD) are now in the market.
Recently, a technique for forming a thin film transistor liquid crystal display fabricated having polysilicon thin film transistors has been developed. The thin film transistor in this type of display has electron mobility much greater than a conventional amorphous silicon (a-Si) type of thin film transistor. Since a display with a smaller thin film transistor and a larger aperture ratio can be produced, a brighter display with lower power consumption is obtained. Moreover, due to the increase in electron mobility, a portion of the driving circuit and the thin film transistor may be fabricated on a glass substrate together at the same time. Thus, reliability and quality of the liquid crystal display panel is improved and the production cost relative to a conventional amorphous silicon type of thin film transistor liquid crystal display is much lower. Furthermore, because polysilicon is a lightweight material with the capacity to produce high-resolution display without consuming too much power, polysilicon thin film transistor display is particularly appropriate for installing on a portable product whose body weight and energy consumption is critical.
Earlier generations of polysilicon thin film transistor were fabricated using solid phase crystallization (SPC) method at temperature higher than 1000° C. With such a high processing temperature, a quartz substrate must be used. Since a quartz substrate costs more than a glass substrate and is also subject to dimensional limitation (not more than 2 to 3 inches in size), polysilicon thin film transistor is only used in small panel display. Now, with the development of laser and maturation of laser crystallization or excimer laser annealing (ELA) techniques, an amorphous silicon film can be easily re-crystallized into polysilicon through a laser scanning operation at a temperature below 600° C. Hence, a glass substrate suitable for forming conventional amorphous silicon TFT-LCD can also be used to fabricate a polysilicon TFT-LCD having larger panel size. Because a lower fabrication temperature is required, this type of polysilicon is often referred to as a low temperature polysilicon (LTPS).
FIGS. 1A to 1C are cross-sectional views showing the progression of steps for fabricating a conventional polysilicon thin film transistor. First, as shown in FIG. 1A, a poly-island layer 102 is formed over a substrate 100. A gate insulation layer 104 is formed over the poly-island layer 102. Because the poly-island layer 102 is formed by recrystallizing an amorphous silicon using a laser crystallization or excimer laser annealing (ELA) process, the poly-island layer 102 contains numerous crystalline defects that often trap mobile electrons.
As shown in FIG. 1B, a gate 106 is formed over the gate insulation layer 104. The gate 106 sits directly on top of that portion of the poly-island layer 102 destined to form a channel region. Thereafter, using the gate 106 as a mask, an ion implantation 108 is carried out to form source/drain regions 102a in the poly-island layer 102 outside the gate-covered region.
To reduce the number of crystalline defects in the poly-island layer 102, a hydrogen-rich silicon oxide layer 110 is formed over aforementioned layers as shown in FIG. 1C. The hydrogen-rich silicon oxide layer 110 is annealed so that the hydrogen atoms within the oxide layer 110 are diffused into the crystalline defects within the poly-island layer 102. In addition, the oxide layer 110 also serves as an inter-layer oxide inside the polysilicon thin film transistor. However, this type of polysilicon thin film transistor has very little leeway for additional improvement of electrical characteristics.